The vacuum casting can suppress the trapping of air in molten metal and this permits obtaining high quality casting with less casting defects such as cavity blanks, blowholes and microporocity. An example of the apparatus to this end is disclosed in Japanese Laid-Open Utility Model Publication No. 3-31058. FIG. 18 shows the apparatus disclosed in the publication.
This apparatus comprises a cavity 2 formed in a casting die 1 and a vacuum pump 4 for reducing the pressure in the cavity 2. The cavity 2 can communicate with a molten metal tank 8, in which molten metal 6 is stored, via a molten metal passage 10. At a sprue 2a of the cavity 2, a gate mechanism 12 is provided for on-off switching the communication between the molten metal passage 10 and the cavity 2. The gate mechanism 12 has an internal axial passage 12b communicated with a pressure reducing pump 20. The passage 12b has an end communicating with the molten metal passage 10 via a vent 12a at a position closer to the molten metal tank 8 than the gate mechanism 12 is.
A molten metal reservoir 14 having a predetermined volume is provided in an intermediate portion of the molten metal passage 10 located to be closer to the molten metal tank 8 than the gate mechanism 12 does.
In this casting apparatus, molten metal is introduced into the cavity 2 in the following way.
First, in a state with the sprue 2a held closed by the gate mechanism 12, the pressure in the cavity 2 is reduced by operating the vacuum pump 4. At this time, the pressure reducing pump 20 is also operated to produce a negative pressure in the molten metal reservoir 14 and molten metal passage 10, thus causing molten metal 6 stored in the molten metal tank 8 to be sucked up to the position of the end of the gate mechanism 12. That is, the molten metal 6 is tentatively stored in the molten metal reservoir 14 and molten metal passage 10.
In this state, the gate mechanism 12 is opened, causing the molten metal 6 stored in the molten metal reservoir 14 and molten metal passage 10 to be sucked and fill the cavity 2.